Digital spread spectrum simulator

ABSTRACT

Simple digital devices and methods are described for reducing electromagnetic interference even where a clock signal to the device is rapidly turned ON and OFF. A primary clock signal drives a signal selector. A signal delay is included to provide delayed clock pulse signals. The signal delay detunes the primary clock signal. By repeatedly switching the delay in and out of the clock signal path, a first signal is generated having a frequency at or about the clock signal, and, a second signal is generated which is displaced slightly from the first signal, but still at or about the clock frequency. By repeatedly switching the delay in and out of the primary clock signal path at a rate greater than the frequency of the clock signal, smaller portions of each primary clock pulse may be parsed for subsequent reaggregation to create a spread spectrum clock signal. As a result, a plurality of spread spectrum clock signals at varying frequencies may be created to spread the energy of the signal over a broader spectrum, thereby lowering the signal energy peak to minimize radiation of electromagnetic emissions at undesirable levels.

REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. application Ser. No. 09/513,538, filed Feb. 25, 2000, now U.S. Pat. No. ______.

FIELD OF THE INVENTION

[0002] This invention relates to electronic circuitry for reducing emissions of electromagnetic interference. More particularly, the invention relates to such circuitry using digital spread spectrum means.

BACKGROUND

[0003] Many electronic devices employ microprocessors or other digital circuits that require one or more clock signals for synchronization. A clock signal permits precise timing of events in the microprocessor. Typical microprocessors may be supervised or synchronized by a free-running oscillator, such as driven by a crystal, an LC-tuned circuit, or an external clock source.

[0004] High performance, microprocessor-based devices using leading edge, high-speed circuits are particularly susceptible to generating and radiating electromagnetic interference (“EMI”). Undesirable EMI is generated when a clock electronic circuit produces a pulsed signal having a pronounced energy peak, including energy at harmonic frequencies. The spectral components of the EMI emissions typically have peak amplitudes at harmonics of the fundamental frequency of the clock circuit. These high-power harmonic electromagnetic pulses can interfere with other carrier signals at higher frequencies, such as radio waves. Accordingly, many regulatory agencies, such as the United States Federal Communications Commission (“FCC”), have established testing procedures and maximum allowable electromagnetic emissions levels for such devices.

[0005] To comply with such EMI limits, costly suppression measures or extensive shielding have been used. Other approaches for reducing EMI have included careful routing of signal traces on printed circuit boards to minimize loops and other potentially radiating structures. Unfortunately, such an approach often leads to more expensive multi-layer circuit boards with internal ground planes. In addition, greater engineering effort must go into reducing EMI emissions.

[0006] In addition, other previously known techniques for reducing EMI use spread spectrum techniques to lower the pronounced energy peak of the clock pulse and still accomplish the desired clock function. Such techniques modulate the clock slowly to spread the energy of the signal over a broader portion of the spectrum about either side of the original peak.

[0007] Several spread spectrum modulation techniques have been employed in the past to minimize EMI from electronic devices, including printers. Many of the latest spread spectrum modulation techniques employ a phase-locked loop (“PLL”) in conjunction with a voltage-controlled oscillator (“VCO”). PLL-based spread spectrum systems rely on analog modulation methods and do not operate reliably when the fundamental clock input to the system is rapidly turned ON and OFF. A separate waveform having a specific profile is overlaid on the waveform of the clock generator, causing a frequency shift during the generation of each clock pulse. A PLL-based system can fail to adequately reduce EMI if the modulated frequency is changed too slowly. Further, PLL-based modulator frequency output can drift due to comparator phase delays, resulting in system jitter and failing to provide a modulated signal with sufficient quality to control or synchronize other elements of the dependent microprocessor or digital circuit.

[0008] There are several other previously known spread spectrum systems. For example, Hardin et al. U.S. Pat. No. 5,488,627 (“Hardin I”) provides a slight variation on the aforementioned PLL-based spread spectrum modulator systems. In Hardin's system, an analog method is used to sweep up and down the reference clock signal frequency to provide the desired modulation. Hardin's method likewise employs a PLL as part of its circuitry. Hardin U.S. Pat. No. 5,631,920 (“Hardin II”), describes another PLL-based spread spectrum modulation approach which is similar to the technique described in Hardin I, but simply uses a different analog method to sweep up and down the reference clock signal frequency.

[0009] Puckette et al. U.S. Pat. No. 5,736,893 (“Puckette”) describes a complex spread spectrum signal modulation system requiring at least one PLL to regenerate the needed frequencies to modulate the clock signal. In a system requiring rapid initialization, Puckette's system would not operate effectively due to the delays associated with ramp-up and synchronization between the input clock signal and the associated reference signal.

[0010] Bassetti et al. U.S. Pat. No. 5,757,338 (“Bassetti”) describes a PLL-based system that uses a spread spectrum modulator for driving a flat panel display and cathode ray tube (“CRT”). In Bassetti's system, the primary purpose is to minimize distortion of displayed images when both a CRT and a flat panel display are used concurrently and in close proximity. A horizontal clock counter is used to modulate the primary reference clock frequency in conjunction with analog components, including digital to analog converters and a voltage-controlled oscillator (“VCO”). Bassetti's system is complex and difficult to adjust for application to varying types of devices other than CRT's and flat panel displays.

[0011] Knierim U.S. Pat. No. 5,659,587 teaches a similar spread spectrum modulation system dependent upon an analog VCO and PLL. Bland U.S. Pat. No. 5,610,955 also teaches an analog VCO-based clock modulator. Each of the above systems include inherent PLL-based delays which would preclude their use in systems, such as printers, where the primary input clock is repeatedly turned ON and OFF.

[0012] Accordingly, a need exists for a simple yet effective electronic apparatus to reduce EMI emissions from microcircuits using clock pulses to regulate their operation. Additionally, a need exists for such an apparatus that is capable of operating during rapid initialization and shutdown of the microcircuit, such as that used in laser printers.

SUMMARY

[0013] The invention provides simple, flexible, and inexpensive devices and methods for reducing EMI. Further, the invention provides circuits that are operable even where the clock signal to the device is rapidly turned ON and OFF. Spread spectrum systems in accordance with this invention generate spread spectrum output clock signals by parsing, sampling, delaying, and reaggregating the various frequency components of an original clock signal. The digital modulation, or spread spectrum simulation, reduces the spectral amplitude of the EMI components at each harmonic of the clock when compared to the spectrum of the same clocking signal without such digital modulation.

[0014] In an exemplary embodiment, electronic circuits in accordance with this invention employ a primary non-modulated clock signal which acts as both the source of the output digitally modulated clock signal and also drives a standard flip-flop. The flip-flop actuates a metal oxide semiconductor (“MOS”) circuit that controls either the operation of a Resistor-Capacitor (“RC”) timing circuit or delay line. The RC timing circuit or delay line detunes the clock signal from the desired nominal frequency. By selecting first the non-modulated clock signal and then the delayed signal, a first signal is generated having a frequency at the clock signal, and a second signal is generated which is displaced slightly from the first signal, but still at the clock frequency. When aggregated, a digitally modulated output signal is produced.

[0015] Detuning the clock signal allows the energy of the clock signal to be sufficiently spread over a broader frequency band to reduce EMI at the primary clock frequency, while still providing desired clock function at the desired nominal frequency for synchronization of various elements of the microcircuit. By repeatedly switching the RC circuit or delay line in and out of the non-modulated clock signal path, a variety of samples of the non-modulated clock signal may be obtained and later aggregated to provide an output clock signal having an approximately equivalent nominal frequency, yet lacking the high energy harmonic pulses that would violate EMI restrictions.

[0016] Other embodiments of the invention include a waveform or clock signal generation means, such as an oscillator, that drives an n-input multiplexer, either directly, or, through n−1 delay lines. As a result, multiple clock signals at varying frequencies may be created to spread the energy of the clock signal over a broader spectrum. Thus, the electromagnetic energy of the clock signal is spread on either side of its original energy peak, thus producing a modulated output clock signal. The modulated output clock signal has a lowered energy peak but retains a nominal frequency equivalent to that of the fundamental frequency of the primary clock signal. Consequently, the circuit being driven, regulated, synchronized or otherwise managed by the modulated clock signal still operates reliably. Hence, by lowering the output signal energy peak at the fundamental frequency and harmonics of that frequency, the individual electronic device is able to meet proscribed FCC regulations.

[0017] Methods and apparatus in accordance with this invention are particularly useful in a high-speed laser printer engine, where it is desirable to have the electronic device quickly and repeatedly initialize and shutdown. Conventional PLL-based spread spectrum modulators are incapable of performing in a system having such instantaneous start and stop requirements. Methods and apparatus in accordance with this invention eliminate the need for the modulation circuit to reach a stable state or ramp up, as required with a PLL-based modulator. Consequently, systems which use a spread spectrum system of the present invention are able to respond more efficiently to changing operating conditions without exceeding EMI restrictions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The above-mentioned objects and features of the present invention can be more clearly understood from the following detailed description considered in conjunction with the following drawings, in which the same reference numerals denote the same elements throughout, and in which:

[0019]FIG. 1 is a circuit diagram of a basic embodiment of the invention using an RC timing circuit;

[0020]FIG. 2a is a block diagram of the spread spectrum circuit having only one delay line according to the invention;

[0021]FIG. 2b is a block diagram of the state machine from FIG. 2a selecting the different inputs to the multiplexer;

[0022]FIG. 3 is a timing diagram providing a discrete snapshot of the operation of the basic spread spectrum circuit having only one delay line according to the invention;

[0023]FIG. 4 is a block diagram of a preferred embodiment of the spread spectrum circuit having two delay lines according to the invention;

[0024]FIG. 5 is a timing diagram providing a discrete snapshot of the operation of the alternative embodiment of the spread spectrum circuit having two delay lines according to the invention;

[0025]FIG. 6 is a diagram of a preferred embodiment of the spread spectrum circuit having two delay lines illustrated in FIG. 4, incorporated within a larger integrated circuit based device according to the invention;

[0026]FIG. 7 is a block diagram of an alternative embodiment of the spread spectrum circuit having three delay lines according to the invention; and

[0027]FIG. 8 is a block diagram of a generic embodiment of the spread spectrum circuit having n delay lines according to the invention.

DETAILED DESCRIPTION

[0028] Referring to the drawings, reference numeral 10 identifies an exemplary embodiment of a digital spread spectrum system according to the invention. FIG. 1 provides a basic block diagram of a first embodiment 10 of the circuit components of the invention. A resistor-capacitor timing circuit 30 and 70 provides one means to detune primary clock signal 22 off its primary frequency.

[0029] In circuit 10, primary clock signal 22 is digitally spread over a broader bandwidth to reduce EMI emissions at the clock fundamental frequency and resulting harmonics of the clock fundamental frequency. Spread spectrum system 10 employs a primary clock signal generator 20 to generate non-modulated primary clock signal 22, which serves as the source for output digitally-modulated clock signal 80, hereinafter identified as spread spectrum signal 80. A delay means 30 with 70, in this case a resistor-capacitor timing circuit, receives the-primary clock signal 22. The signal 32 is delayed when MOS switch 60 is turned ON, effectively connecting capacitor 70 to resistor 30, or not delayed when MOS switch 60 is turned OFF, isolating capacitor 70 from resistor 30.

[0030] Primary clock signal 22 is routed through frequency divider 52, which outputs signal 54 at half the primary clock frequency 20. Divider 54 actuates an n-channel MOS (“NMOS”) switch 60 that connects or isolates capacitor 70 from resistor 30. Although shown as timing delay circuit 30 and 70, the delay means-may be an RC circuit 30 and 70, a delay line 131 (FIG. 2a) or some other similar device or circuit capable of selecting and delaying transmission of an inputted signal. RC circuit 30 and 70 (FIG. 1) and delay line 131 (FIG. 2a) both provide a means to detune primary clock signal 22 from its primary nominal frequency. By repeatedly switching capacitor 70 circuit (FIG. 1) in and out of signal path 32, or delay line 131 (FIG. 2a) in and out of primary clock signal 22 path, two differently timed signals may be generated.

[0031] Referring once again to FIG. 1, first non-delayed signal 32 is generated having a frequency at primary clock signal 22, and, second delayed signal 32 is generated which is displaced slightly from the frequency of first signal 62, but still at the frequency of primary clock signal 22. De-tuning primary clock signal 22 allows the energy of each pulse of primary clock signal 22 to be sufficiently spread over a broader frequency band to reduce the EMI at the fundamental frequency of primary clock signal 22, while still providing desired clock function at a desired nominal clock frequency. For the purposes of this description, the nominal clock frequency is that frequency resulting from the re-aggregation of a plurality of parsed spectral components of primary clock signal 22 to form a new spread spectrum clock signal 80 having different pulse amplitude and spectral spreading. Generally, the nominal frequency is sufficiently similar to the fundamental frequency of primary clock signal 22 that the devices being controlled or synchronized by system clock 20 still operate correctly when driven by spread spectrum signal 80 having such a nominal frequency. The generated nominal frequency has a lower spectral amplitude than the fundamental frequency of primary clock signal 22 due to the spectral spreading of primary clock signal 22.

[0032] More particularly, and with specific reference to FIG. 1, system 10 is described. A clock generating means 20 produces a non-modulated clock signal 22. Clock generating means 20 may include a VCO, a piezoelectric crystal or other such similar components capable of producing a series of regular and stable clock pulses at a frequency used to drive and synchronize other elements of a microcircuit-based device. For purposes of explaining the operation of the invention, primary clock signal 22 is presumed to have a frequency of 66 MHz, which corresponds to the operational frequency of many of today's popular microprocessors. However, one skilled in the art will recognize that the invention may be used to digitally spread any clock signal 22 at any frequency. Digital system boards currently operate at speeds in excess of 100 MHz. The invention is readily adaptable to such high speed and future higher speed devices that may be developed.

[0033] As shown in FIG. 1, primary non-delayed clock signal 22 may be first routed through frequency divider 52. For purposes of this example, frequency divider 52 is shown as a divide-by-two frequency divider 52. However, other frequency dividers or finite state machines may be used to accommodate differing digital circuit applications. In this example, frequency divider 52 divides an originating 66 MHz clock signal 22 to generate a divided 33 MHz clock signal 53. The 33 MHz clock signal 53 drives a divide-by-two flip-flop 54. Flip-flop 54 in turn drives NMOS switch 60, which regulates connection of the capacitor.

[0034] By driving flip-flop 54 at half the frequency, NMOS switch 60 is caused to alternate every other clock period between non-delayed clock signal 32 and the delayed clock signal at a rate parsing one pulse into two spectral components. Concurrently, the parsed pulses may then be reaggregated to create a digitally spread output clock signal pulse, resulting in a spread spectrum signal 80. The period of each digitally spread clock signal pulse is alternatively longer or shorter than that of the original dock pulse. The length of the spread pulse is determined by delay time D of the RC timing circuit 30. The length of signal delay D is controlled by the operating characteristics of the selected resistor circuit 30 and the capacitance of capacitor 70.

[0035] Again, by driving flip-flop 54, and hence, NMOS switch 60 at half the clock frequency of primary clock signal 22 frequency, spread spectrum system 10 generates a spread spectrum output pulse signal 80 composed of portions of the original clock pulse and portions of the original clock pulse delayed by the amount of delay time D inherent within RC timing circuit 30 and 70. The period T of the pulse is expanded or contracted by the amount of delay time D inherent within AC timing circuit 30, while the total energy of the pulse remains substantially constant. Consequently, having spread the energy of a single clock pulse over a period of T+D and T−D, while the total pulse energy remains constant, the amplitude of the outputted spread spectrum pulse is lower at both the nominal frequency and harmonics of that frequency. Hence, by varying the delay time D of RC timing circuit 30 and 70, the spread in frequency of signal 80 can be adjusted with infinite granularity. The generated spread spectrum clock signal 80 provides desired system synchronization via generation of a specific nominal frequency while minimizing radiation of undesirable EMI.

[0036] Referring now to FIG. 2a, an alternative embodiment 110 of the spread spectrum system is described. In this embodiment, switched capacitor timing circuit 30 and 70 (FIG. 1) is replaced with a static RC or chip-level delay line to provide a means to delay primary clock signal 122. In addition, a single delay line having a delay time of D/L1 receives an input of the primary clock signal 122. Spread spectrum system 110 may select from either the original non-delayed clock input signal 122 or the delayed signal 131. Both a clean line carrying original reference clock signal 122 and the delay line carrying detuned clock signal 131 are routed to a two-to-one multiplexer 140, which includes a primary input pin to receive primary clock signal 122 and a first input pin A 144 to receive the delayed clock signal from the first delay line.

[0037] A state machine 150 receives an input from original primary clock signal 122 which synchronizes operation of state machine 150 with operation of multiplexer 140. In this second embodiment 110, state machine 150 generates three states 111 (FIG. 2b) sequentially inputted to multiplexer 140 via a line 152. FIG. 2b is a block diagram of the state machine from FIG. 2a selecting the different inputs to the multiplexer

[0038] Referring now to both FIGS. 2a and 3, the timing diagram provided in FIG. 3 illustrates the discrete operation of single delay line embodiment 110 of the spread spectrum system according to the invention. The timing diagram shows the original non-delayed clock signal 122 and delayed clock signal 131 as each having equal periods of 1. Delay line 130 has an inherent delay time of D/L1. When flip-flop 140 of the 2-bit state machine 150 is low, it issues a low signal 152 on the SEL pin which causes multiplexer 140 to select and output non-delayed signal 122 present at the primary clock signal input pin CLK 142. When flip-flop 140 of the 2 bit state machine 150 is high, it issues a high signal 152 on the SEL pin which causes multiplexer 140 to select and output delayed signal 131 present at delay line input pin A 144.

[0039] State machine 150 and multiplexer 140 cooperate to parse, sample, and reaggregate original clock signal 122 and delayed signal 131 to provide an output spread spectrum clock signal 180 whose period switches alternatively between T, T+DL1, T−DL1 decreasing the peak energy at the main frequency of f=1/T of the primary clock signal. The total energy is effectively spread to three frequencies at f1=1/T, f2=1/(T+DL1) and f3=1/(T−DL1). Thus, depending on input original clock signal 122, the delay time D/L1 inherent in delay line 130, the selection of multiplexer 140 input by the state machine 150, one is able to design a spread spectrum system circuit configuration according to the invention that is capable of generating a plurality of different aggregate output spread spectrum clock signals 180.

[0040] In an alternative embodiment 210 according to the invention (FIG. 4), two delay lines 230, 232 are provided having inherent delay times of D1 and D2, respectively. For simplicity of description, the delay time of first delay line 230 is D1, the delay time D2 of the second delay line 232 is two times the delay time of first delay time 230. Hence, D2 is equal to D1+D1.

[0041] However, as shown in FIG. 4, the output of first delay line 230 is routed to both multiplexer 240 and second delay line 232. Consequently, the actual delay time of second delay line 232 is a sum of delay time D1 inherent in first delay time 232 and delay time D2 in second delay line 232. Thus, a signal outputted from second delay line 232 is actually delayed by 3*D1. This particular delay line combination is but one of a plurality of different delay line configurations contemplated by the invention which may be modified to generate a plethora of differing outputted spread spectrum signals 280.

[0042] This dual delay line embodiment 210 of the spread spectrum system is thus capable of generating three signals: the original non-delayed clock signal 222, the D1 clock signal 231, and the D3 clock signal 233. Each clock signal 222, 231, 233 is routed to separate input pins 242, 244, 246 of a 3 to 1 multiplexer 240, which is then able to sample each of signals 222, 231, 233. Accordingly, viewing the discrete operation of the dual delay line spread spectrum, original non-delayed clock signal 222 is parsed, sampled, and reaggregated to provide output spread spectrum signal 280 having a period of T+D1+D2 or T+3*D1. Consequently, the period of outputted spread spectrum signal 180 is lengthened or reduced while the total energy of the signal remains constant. Accordingly, the amplitude of the signal at all frequencies is substantially lowered, thereby reducing the level of EMI produced at the nominal frequency and all harmonics thereof during the generation of the signal. The timing diagram associated with the discrete operation of this preferred embodiment is provided in FIG. 5 to illustrate how the addition of one more delay line substantially increases the number of possible configurations for the resulting spread spectrum signal 280.

[0043] Referring to FIG. 6, an example of spread spectrum systems in accordance with the invention is shown integrated within a larger digital device 290, such as a field programmable gate array (“FGPA”) or an application specific integrated circuit (“ASIC”). Given the ability of spread spectrum circuit 210 to use extremely small delay line technology, the spread spectrum circuit 210 can be incorporated on an FGPA, an ASIC or other digital device. As further shown in FIG. 6, digital device 290 can include other logic 292, fixed or programmable, that can use spread spectrum signal 280 within the digital device. For example, spread spectrum signal 280 can be directed to provide a modulated data bus 282, modulated control signals, 284 or modulated auxiliary clocks 286. Consequently, spread spectrum system 210 may be used to generate a plurality of different spread spectrum signals 280, 282, 284, 286 for use in controlling and synchronizing various elements of the microcircuit-based device. Additionally, such integration of spread spectrum signal system 210 within a larger digital device or integrated circuit 290 allows more on-chip logic 292 to be used to further control the output of the state machine, thereby potentially increasing the number of available selection sequences to generate spread spectrum output signal 280. Therefore, a primary clock signal 222 may be modified to create a plurality of different aggregate output signals 280 that uniquely adapt to the requirements of a particular digital device.

[0044] Hence, unlike PLL-based clock pulse signal modulation systems, where the modulated output signal is the result of the overlay of a particular waveform profile over the waveform generated by the clock, the invention can generate multiple output spread spectrum clock signals 280 from a single primary clock signal 222 to serve different purposes.

[0045] Referring now to FIG. 7, spread spectrum systems in accordance with this invention provide additional unique design advantages over and above the selection and aggregation of individual multiplexer inputs. For example, in an alternative embodiment 310, where multiple delay lines 330, 332, 334, having differing delay times D1, D3, D6, may be selected and multiplexed to create the modulated spread spectrum clock output signal 380, the different delay lines 330, 332, 334 are preferably selected as a suite of delays D1. In one scenario, state machine 350 is programmed to generate control signals 352 which cause 4-to-1 multiplexer 340 to select inputs 342, 344, 346, 348 in a manner which allows a single individual inputs to be selected or multiple inputs to be selected simultaneously. A designer would be able to program logic so as to select delays where the number of different frequencies is maximized.

[0046] For example, alternative embodiment 310 shown in FIG. 7 includes three delay lines inputs 330, 332, 334 and one original clock signal input 322 routed to the multiplexer 340. Where the original clock period is T, first delay line 330 includes a delay time of D1=1, second delay line 332 includes a delay time of D3 equal to three times D1 or 3 and third delay line 334 includes a delay time of D6 equal to six times D1 or 6, spread spectrum system 310 is able to generate the following discrete clock signal periods:

[0047] a) T;

[0048] b) T+1, T+2, T+3, T+5, T+6;

[0049] c) T−1, T−2, T−3, T−5, and T−6.

[0050] The varying periods result in the generation of an equivalent number of clock frequencies which may be reaggregated to create a desired spread spectrum signal 380. Hence, referring to FIG. 7, by incorporating three delay lines 330, 332, 334 and one clean original clock signal line 322 in the circuitry, a total of eleven different signal frequencies may be generated. These signals may then be aggregated by the multiplexer 340 to more uniformly spread original clock signal 322 over a greater aggregate bandwidth. Increasing the number of available frequencies for reaggregation tends to reduce the spectral noise.

[0051] Correspondingly, where four delay lines are included in spread spectrum system, providing a total of five different clock signal inputs to the multiplexer, a total of 5*5−6=19 frequencies may be generated. Similarly, where five delay lines are included in the spread spectrum system, providing a total of six different clock signal inputs made available to the multiplexer, 6*6−8=28 frequencies may be generated. By selecting the different delays as a suite in the following manner: T1=1, T2=1+2, T3=1+2+3 etc . . . , the number of different frequencies is maximized. Consequently, there is no theoretical limit to the number of frequencies that may be generated by the spread spectrum system of the invention. The limit of the number of frequencies that may be generated is controlled only by the ability to incorporate a number of different delay lines and large multiplexers on board a microprocessor or other digital circuit.

[0052] Consequently, one skilled in the art will readily recognize that the implementation of the spread spectrum system may be adapted to provide more complex and differing spread spectrum signals as different technology is developed to reduce the size of delay lines, multiplexers, flip-flops and other elements of the spread spectrum system. Additionally, one skilled in the art will recognize that other algorithms may be incorporated in the logic of the spread spectrum system to generate outputted spread spectrum signals having different aggregate frequency profiles and amplitude. For example, modifying programmable logic used to control the operation of the multiplexer could provide for selecting delays in a differing suite or in a different order.

[0053] The delay lines need not be selected in sequence, starting with the original non-delayed clock signal. A first delay line may be selected by the multiplexer after the selection of the second delay line. As a result, as most clearly shown in discrete operation of the invention depicted in FIG. 5, the spread between signals can be modified and varied substantially. The greater the number of delay lines, the greater the number of available combinatorial overlays of the frequencies and the greater the number of available spacing between the individual and combined frequencies. Hence, the unique ability of spread spectrum system 310 to generate a plethora of available signals allows the signals to be aggregated in a number of different ways to generate an even more complex spread spectrum signal 380.

[0054] Hence, a further benefit of the spread spectrum system of the invention is the ability to use one standard spread spectrum circuit configuration to generate a plurality of different spread spectrum signals. This advantage can reduce overall microcircuit-based device cost since standard spread spectrum circuit configurations could be used for many different applications.

[0055] As evidenced by the above descriptions and generically illustrated in FIG. 8, one skilled in the art will readily recognize that spread spectrum system 510 according to the invention could be extended to include an infinite number, n, of delay lines 530, 532, 534, 536 providing n+1 selectable multiplexer inputs and resulting in a total output spread spectrum signal period of T+D1+D2 . . . Dn. In practice, the number and delay period D of delay lines are adjusted to accommodate the specific needs of a particular microcircuit. The number of delay lines could be extended until the original clock signal 522 is spread across a period of sufficient length that the nominal frequency is unintelligible or of insufficient energy to provide the desired synchronization for the system in which it is employed. One skilled in the art will also recognize that spread spectrum system 510 may be implemented using any of a plurality of different multiplexers 540, state machines 550, and clock pulse generation means 520. The system and method of the present invention may be implemented using technology that is currently available, and, will accommodate future technology operating at much higher speeds.

[0056] Unlike existing spread spectrum modulators, spread spectrum system 10 provides a unique ability to immediately respond to an input clock signal and create a desired output spread spectrum signal. Consequently the circuit of the invention uniquely lends itself to use in low power devices which may require rapid initialization and shutdown. Such devices might be used in high speed printers that require to enter lower power mode required in office environment. Hence, the spread spectrum system reduces device latency while still providing for compliance with EMI regulatory restrictions. Consequently, EMI suppression systems of the present invention might be used in a substantially greater number of high speed devices, thereby improving efficiency and response of the overall integrated system and providing for broader consumer use of high speed microcircuit-based devices.

[0057] The invention particularly and uniquely lends itself to uses where it is desirable to have the electronic device initialize and shutdown quickly and repeatedly. For example, high speed laser printers may be driven by video card components which are repeatedly turned ON and OFF whenever a demand is placed on the printer to produce printed output. During this initialization period, a conventional PLL-based system would need to stabilize before being able to provide a desired clock signal to synchronize the operation of the printer. Hence, print response time is delayed. In addition, a conventional PLL-based spread spectrum signal modulation system might generate undesirable EMI during its stabilization period. The invention eliminates the need for the circuit to reach a stable state or ramp up. Once the first clock signal is generated, the spread spectrum system is able to generate a desired spread spectrum signal. Consequently, systems which use the present invention are able to respond more efficiently to changing operating conditions without exceeding EMI restrictions. Further, the invention eliminates the need to design for stray EMI generated during the startup period. The enhanced response time of the invention helps to improve overall system utility since one delay or bottleneck can be eliminated.

[0058] As will be readily understood by those skilled in the art, in an implementation of any of the circuits described herein in a physical package, such as a FGPA or ASIC, several such spread spectrum circuits may be incorporated in the same physical package. The spread spectrum system may also be included internally with a microprocessor or any other digital circuit.

[0059] The above described circuit configurations support implementation of methods according to the invention for generating spread spectrum clock output signals. Methods of the invention preferably include the steps of: generating a series of clock pulses, then digitally modulating those clock pulses by parsing individual clock pulses into a plurality of portions, sampling the portions, and then reaggregating the portions to create a spread spectrum clock pulse signal having a broader bandwidth and flatter signal amplitudes. The ability to reaggregate and customize the clock signal allows amplitude at harmonics of the fundamental clock frequency to be lowered, thereby minimizing generation of undesirable EMI spectral components which would otherwise be produced along with the series of primary clock pulses. The step of digitally spread spectrum modulating the series of clock pulses preferably includes the step of creating a digital spread spectrum simulation of each individual pulse by parsing, selecting, combining and reaggregating the clock pulses via the novel circuits of the various embodiments of the spread spectrum system, as described in greater detail above.

[0060] Methods of the invention include the step of providing a means for generating a series of clock pulses. The clock generation means may include a voltage controlled oscillator, a piezoelectric crystal or other such devices capable of generating a series of clock pulses.

[0061] A next step includes providing a means for delaying transmission of a clock pulse or a portion of a clock pulse. Such delay means includes a resistor-capacitor timing circuit, a delay line or other such devices capable of delaying the transmission of a clock signal through an electronic circuit.

[0062] A further step includes providing a means for selecting one of a plurality of clock signals, wherein the clock signals have varying delay times. Such selection means include NMOS switches driven by flip-flops, multiplexers controlled by the output of a state machine and other such devices capable of selecting one of a plurality of signals. Such selection means may also include the capability to combine one or more selected clock signals or portions thereof to create a new clock signal having the characteristics of the combined clock signals or portions thereof.

[0063] An additional step includes providing a means to control the sequence of selection and method of recombination of the selected clock signals or portions to provide an aggregate spread spectrum output clock signal pulse comprised of the components of the selected and combined clock signals.

[0064] This disclosure is provided to reveal a preferred embodiment of the invention and a best mode for practicing the invention. However, one skilled in the art will readily appreciate that other combinations of circuitry components may be substituted for those set forth herein without departing from the spirit and scope of the present invention. Further, additional advantages, applications and modifications of the invention will readily occur to those skilled in the art. Accordingly, the invention should only be limited by the claims included below. 

1. A spread spectrum system for generating a reduced amplitude clock pulse from an original primary clock pulse, the system comprising: a clock signal generator for creating a series of clock pulses; a non-delayed line adapted to receive the series of clock pulses from the clock signal generator; a delay line comprising a delay time and adapted to cause a clock signal transmitted to the delay line to be outputted after passage of the delay time; and a multiplexer comprising a non-delay line input, a delay line input and an output, wherein the multiplexer receives output directly from the non-delayed line and the delay line; and a state machine adapted to cause the multiplexer to select either the non-delayed line or the delayed line.
 2. A method for generating a clock output signal with reduced amplitude electromagnetic interference (EMI) spectral components, the method comprising: providing a means to generate a series of clock pulses; generating a series of clock pulses; digitally modulating the clock pulses by spreading the energy of each clock pulse over a greater bandwidth; and generating a digitally-modulated clock output signal.
 3. A clock circuit for generating a clock output signal with reduced amplitude electromagnetic interference (EMI) spectral components, the clock circuit comprising: oscillator means for generating a primary clock signal, wherein the primary clock signal has a fundamental frequency with a fundamental amplitude; and spread spectrum clock generating means cooperating with the oscillator means for generating a spread spectrum clock output signal having a nominal frequency and reduced amplitude EMI spectral components at harmonics of the nominal frequency, wherein the nominal frequency is substantially equivalent to the fundamental frequency, and wherein the nominal amplitude is lower than the fundamental amplitude. 